1. Field of the Invention
This invention relates to a system and method for rebalancing a battery during vehicle operation.
2. Background Art
Conventional electric (EV), hybrid electric (HEV), and fuel cell (FC) vehicles generally include one or more batteries. Each battery is generally composed of a plurality of electrochemical cells (i.e., cells) combined in series to produce a potential. The smallest unit (i.e., grouping) of cells for which a state of charge (SOC) may be determined is generally referred to as a module.
The cells and/or modules of the battery are generally matched by capacities and voltages during the battery fabrication process. The fabrication process may also include charging and discharging the battery to confirm that all module voltages are within a specified tolerance at all voltage levels. The assembly process generally provides a battery having module voltages within a few milli-volts of one another.
Over time, ambient conditions and/or charging/discharging the battery during vehicle operation may result in differences among the individual SOC of the cells (SOCCell). The SOCCell variations are generally observed as divergence in module voltages (i.e., states of charge) since a module is generally the smallest grouping (i.e., unit) of cells for which a state of charge (SOC) may be determined.
Conventionally, the batteries are controlled between predefined minimum and maximum SOC limits to prevent over-discharge and over-charge of any cell within the battery. When a module approaches the minimum SOC limit, the battery discharge current is driven to zero. When a module approaches the maximum SOC limit, the battery charge current is driven to zero. Thus, any divergence among SOC modules may reduce the operating range of the battery which may, in turn, result in reduced battery performance and a reduced battery life. Accordingly, it is generally desirable to rebalance the battery when a divergence among module SOC is identified.
Conventional methods for rebalancing the battery (i.e., reducing or eliminating module SOC divergence) require low-rate constant-current overcharge of the cells/modules. Such low-rate constant-current overcharge generally requires precise control of charge current (e.g., 1 to 2 Amperes) and may require a long period of time (e.g., 5 hours or more) to complete.
Conventional hybrid and fuel cell vehicle charge-control systems are generally not configured to control the charge current within the conventional rebalancing range during vehicle operation. Furthermore, during recharging of the cells (i.e., battery recharging), the vehicle may experience decreased operating performance (e.g., poor vehicle acceleration, reduced fuel economy, etc.). The decreased vehicle performance may result in a vehicle operator perceiving different or unusual vehicle behavior.
Conventional recharging of a pure electric vehicle generally rebalances the cells of the EV battery. Such rebalancing of EV batteries is generally possible because the vehicle is plugged into a steady power supply (e.g., the United States power grid) and allowed to slowly charge to a full SOC during EV recharging. The slow charging process inherently rebalances the SOC of the cells. However, rapid EV recharge techniques are under development which may reduce the inherent rebalancing of an EV battery during EV recharging. Accordingly, purely electric vehicles may also require a system and method for rebalancing a battery during vehicle operation.
Therefore, a system and method for rebalancing a battery (i.e., the cells of the battery, the modules of the battery, etc.) during vehicle operation may be desirable. Furthermore, a system and method for rebalancing a battery during vehicle operation that reduces and/or eliminates recharge related vehicle performance degradation and/or improves control of the charge current may be desirable.